1. Field of the Invention
The present invention relates to an electric motor used on board a vehicle, and more particularly, to a motor armature shaft support structure for use in the motor.
2. Description of the Related Art
When an armature shaft is rotatably supported by bearings on a yoke or a bracket in this type of motor, axial and radial displacements of the bearings need to be restricted. Particularly in a motor for driving a piston used in an anti-lock braking device in which a radial load may act on its armature shaft, consideration should be given to restricting the radial motion of bearings. As disclosed in Japanese Unexamined Patent Publication No. 7184344, grooves are provided where the outer ring of bearings and the housing of the bearings are installed, and resin is applied into the grooves to restrict the axial and radial motions of the bearings. In this method, however, resin must have a sufficient durability. With a long period of service, the resin ages, thereby reducing the effectiveness of the resin in restricting movement of the bearings. Furthermore, an additional manufacturing step for inserting resin into the grooves is required, lowering the efficiency of production.
The inventor of this invention has learned that the problem of the axial and radial displacements of an armature shaft is resolved by press-fitting the armature shaft into the inner ring of bearings with the outer ring of bearings press-fitted in the yoke and bracket.
As shown in FIG. 10, some motors employ a cylindrical yoke 2 opened at one end and closed at the other end, and set bolts 16 are screwed in from the side of the closed end of the yoke 2a to secure the yoke 2 to a bracket 3. The outer ring 9b of bearings 9 is press-fitted into the yoke 2 while an armature shaft 4 is press-fitted into the inner ring 9a of the bearings 9. This arrangement suffers the following problem.
When the outer ring 9b of the bearings 9 is press-fitted into a bearing housing 2b formed at the closed end 2a of the yoke 2, stress in excess of permissible level may be radially applied to the bearings 9. Since the bearings 9 are ball bearings, they may be damaged under such excess radial stress and may malfunction. For this reason, the press-fit load on the outer ring 9b, (P1,) needs to be smaller than the permissible radial load, PR (thus, P1&lt;PR). Since the dimensions of the bearing housing 2b vary within a rated tolerance, the press-fit load P1 of the outer ring 9b is different from product to product within a range below the permissible radial load PR. With the outer ring 9b press-fitted, it remains abutting the closed end 2c of the bearing housing 2b (see FIG. 12).
Then, the armature shaft 4 is press-fitted into the inner ring 9a of the bearings 9 already press-fitted into the bearing housing 2b, using a jig E that may be inserted from the bearing housing closed end 2c, as shown in FIG. 11. If the press-fit load P2 of the inner ring 9a is greater than the permissible axial load PA of the bearings 9, a load in excess of the permissible axial load PA acts on the bearings 9 when the opened end of the yoke 2 abuts the bracket 3, and the bearings 9 may be damaged. The press-fit load P2 of the inner ring 9a must be set smaller than the permissible axial load PA (P2&lt;PA).
In conventional electric motors, the assembly of the inner and outer rings 9a, 9b of the bearings 9, the yoke 2 and the armature shaft 4 suffers variations in the relationship of design parameters as follows.
a) Press-fit load of inner ring&lt;press-fit load of outer ring&lt;constant pressure load of press&lt;permissible axial load (P2&lt;P1&lt;PP&lt;PA).
b) Press-fit load of outer ring&lt;press-fit load of inner ring&lt;constant pressure load of press&lt;permissible axial load (P1&lt;P2&lt;PP&lt;PA).
c) Press-fit load of inner ring&lt;constant pressure load of press&lt;press-fit load of outer ring&lt;permissible axial load (P2&lt;PP&lt;P1&lt;PA).
d) Press-fit load of inner ring&lt;constant pressure load of press&lt;permissible axial load&lt;press-fit load of outer ring (P2&lt;PP&lt;PA&lt;P1)
The constant pressure load PP of a press or jig means a load limit involved in the press-fitting of the inner ring 9a of the bearings 9, and is set to be greater than the press-fit load P2 of the inner ring 9a but smaller than the permissible axial load PA of the bearings (thus, P2&lt;PP&lt;PA).
In a) and b) of the above cases a)-d), the press-fit load P1 of the outer ring 9b is smaller than the constant pressure load PP of the press (P1&lt;PP). Now, the inner ring 9a is press-fitted around the armature shaft 4. When the inner ring 9a is further pressed even after the open end of the yoke 2 abuts the bracket 3, the outer ring 9b moves along with the inner ring 9a toward the open end of the yoke, because the press-fit load P1 of the outer ring 9b is smaller than the constant pressure load PP. This movement is stopped by a step portion 4b formed on the armature shaft 4 when the inner ring 9a touches it, and thus pressing of the inner ring 9a by the jig E stops when the constant pressure load PP of the press is reached. A gap S is left between the closed end 2c of the bearing housing 2 and the outer ring 9b, because the bearings 9 are shifted toward the open end of the yoke 2 (see FIG. 13(X)).
When the set bolts 16 are tightened by a constantly controlled torque to secure the yoke to the bracket 3, the yoke 2 is deformed to be shortened in its axial length. The quantity of deformation D may be greater than the gap S (D&gt;S). If it is so, the outer ring 9b of the bearings 9 is urged toward the yoke open end by the closed end 2c of the bearing housing, though the inner ring 9a is already in contact with the step portion 4b and is unable to move. A load above the permissible axial load PA may act between the inner ring 9a and the outer ring 9b, possibly breaking the bearings 9 (see FIG. 13(Y)).
In cases c) and d), the press-fit load P1 of the outer ring 9b is greater than the constant pressure load PP of the press (P1&gt;PP). In the press-fitting of the inner ring 9a around the armature shaft, the inner ring 9a may be further pressed after the open end of the yoke 2 abuts the bracket 3. Since the press-fit load P1 of the outer ring 9b is greater than the constant pressure load PP of the press, the outer ring 9b remains still, and thus neither the inner ring 9a nor the outer ring 9b moves. When the constant pressure load PP is reached, the pressing of the inner ring 9a by the jig E is stopped. The gap S is left between the inner ring 9a and the step portion 4b of the armature shaft 4 (see FIG. 14(X)).
When the set bolts 16 are tightened by a constantly controlled torque to secure the yoke to the bracket 3, the yoke 2 is deformed to be shortened in its axial length. The outer ring 9b of the bearings 9 is urged toward the yoke open end by the closed end 2c of the bearing housing, and thus both the inner ring 9a and the outer ring 9b are moved toward the yoke open end. If the deformation of the yoke 2 is greater than the gap S (D&gt;S), the outer ring 9b is urged by the closed end 2c of the bearing housing toward the yoke open end even after the inner ring 9a touches and is then restricted by the step portion 4b of the armature shaft 4. As a result, a load above the permissible axial load PA may act between the inner ring 9a and the outer ring 9b, possibly breaking the bearings 9 (see FIG. 14(Y)).
In any of the cases a) through d), there is a possibility that a load above the permissible axial load PA of the bearings acts when the set bolts are tightened. To resolve this problem, after the outer ring is press-fitted into the bearing housing, the side of the outer ring of the bearings toward the yoke open end is secured to the yoke through caulking, for example, with the outer ring abutting the closed end of the bearing housing so that the outer ring does not move during the press-fitting of the inner ring around the armature shaft. In this method, however, an additional manufacturing step of caulking is required, lowering the efficiency of production.